Optical storage media are media in which data are stored in an optically readable manner, for example by means of a laser and an optical detector, for example a photo detector, being integrated within a pickup. The detector is used for detecting reflected light of the laser beam when reading data on the storage medium. In the meanwhile a large variety of optical storage media are known, which are operated with different laser wavelength, and which have different sizes for providing storage capacities from below one Gigabyte up to 50 Gigabyte (GB). The formats include read-only formats such as Audio CD and Video DVD, write-once optical media such as CD-R and DVD-R, DVD+R, as well as rewritable formats like CD-RW, DVD-RW and DVD+RW. Digital data are stored on these media along tracks in one or more layers of the media.
The storage medium with the highest data capacity is at present the Blu-Ray disc (BD), which allows to store up to about 50 GB on a dual layer disc. For reading and writing of a Blu-Ray disc an optical pickup with a laser wavelength of 405 nm and a numerical aperture of 0.85 is used. On the Blu-Ray disc a track pitch of 320 nm and a mark length from 2T to 8T or 9T is used, where T is the channel bit length and wherein 2T corresponds with a minimum mark length of 138, 149 or 160 nm.
The diffraction limit of optical instruments as described by the Abbe theory is about lambda/2NA, which is 238 nm for a Blu-Ray type pickup with a wavelength lambda=405 nm and a numerical aperture NA=0.85. This theoretical minimal detectable length from the diffraction theory is corresponding to a period of the pattern function, which is formed of a pit and of a land having the same length. The smallest detectable element of such a system is a pit or a land having a length of about lambda/4NA, which corresponds for a Blu-Ray type pickup with a length of 120 nm.
New optical storage media with a super-resolution structure offer the possibility to increase the data density of the optical storage medium by a factor of two to four in one dimension as compared with the Blu-Ray disc. This is possible by including a nonlinear layer, which is placed above a data layer of the optical storage medium, and which significantly reduces the effective size of a light spot used for reading from or writing to the optical storage medium. The nonlinear layer can be understood as a mask layer because it is arranged above the data layer and for some specific materials only the high intensity center part of a laser beam can penetrate the mask layer. Further, semiconductor materials can be used as a nonlinear layer, e.g. InSb, which show a higher reflectivity in the center part of the focused laser beam, and which center reflectivity is dependent on the pit structure of the corresponding data layer. Therefore, the super-resolution effect allows to record and read data stored in marks of an optical disc, which have a size below the optical resolution limit of lambda/4NA of a corresponding optical pickup.
The nonlinear layer is often called a super-resolution near-field structure (Super-RENS) layer because it is assumed that for some specific materials, the optical effect of reducing the effective spot size of the laser beam is based on a near-field interaction between the marks and spaces of the data layer and the nonlinear layer. Super-RENS optical discs comprising a super-resolution near-field structure formed of a metal oxide, a polymer compound or a phase-change layer comprising GeSbTe or AgInSbTe are known.
In EP-A-1215665, an optical storage medium is disclosed comprising a read-only data layer having a non-inverse pit/land data structure with concave pits corresponding with holes and an optical storage medium comprising a read-only data layer having an inverse pit/land data structure with convex pits corresponding with bumps on a substrate layer.
WO 2009/109614 and WO 2009/109653 disclose an optical disc comprising a substrate layer, a read-only data layer arranged in tracks on the substrate layer and a non-linear layer with a super-resolution structure disposed on the data layer. An inversed data structure is proposed for the smallest pits and lands corresponding with super-resolution pits and lands having a size below the optical resolution limit of an optical pickup for reading of the data, to overcome a problem that the smallest pits and lands provide an inverted read-out signal when reading the data on the optical disc. This problem has emerged in particular when using a phase change material as the super-resolution structure.
EP-A-814 464 describes an optical disc comprising marks and spaces of different lengths, wherein the shortest marks of a mark train have a width larger than the other marks of the mark train. In another embodiment, pits having different lengths have different widths such, that the width is inversely proportional to the length of the pits as long as the length is equal to or less than the diameter of a reproducing beam spot of a corresponding pickup for reading of the data.
KURIHARA ET AL: “High-Speed Fabrication of Super-Resolution Near-Field Structure Read-Only Memory Master Disc using PtOx Thermal Decomposition Lithography”, JAPANESE JOURNAL OF APLLIED PHYSICS, vol. 45, no. 2B, 24 Feb. 2006 (2006-02-24), pages 1379-1382, describes investigations on super-resolution near-field structure read-only discs, on which pits of different shapes are provided, and its effects on the super-resolution near-field structure disc properties where studied. It was found that elliptical pits with a high aspect ratio exhibit enhanced readout characteristics with regard to round pits, evaluated with regard to the carrier-to-noise ratio.
KIM ET AL: “Random Signal Characteristics of Super Resolution Near Field Structure Read Only Memory Disc”, JAPANESE JOURNAL OF APLLIED PHYSICS, vol. 45, no. 2B, 24 Feb. 2006 (2006-02-24), pages 1374-1378, describes super-resolution near-field structure optical discs having pits of different shapes, and noise characteristics have been measured as a function of the pit width. It was found that the readout characteristics of short pits below the optical resolution limit is improved when increasing the pit width.